Structure of Atom

Atoms are basic building blocks of matter, the word atom is derived from the Greek word atom which simply means indivisible, most Greek philosophers thought about something in nature that is indivisible later the word used for the first discovered tiny particle like an atom.
Atoms are composed of three types of particles which we knew as electron, protons and neutrons. Protons and neutrons are responsible for most of the atomic mass. The mass of an electron is very small (9.108 x 10^28 grams) negligible than the mass of a proton. Both the protons and neutrons reside in the nucleus. Protons have a positive (+) charge, neutrons have no charge(0) they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-). It is the number of protons that determines the atomic number. The number of protons in an element is constant, whereas the neutron number may vary, so the mass numbers (protons + neutrons) may vary. The same element may contain different numbers of neutrons, these forms of an element are called isotopes.

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The chemical properties of isotopes remain same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form.

ISOBARS-: Isobars are atoms of different elements which have the same mass number but a different atomic number.

Ex- Argon(Ar) and Calcium(Ca) have same mass number 40 but different atomic number 18,20 respectively.